Telecommunications access networks have significantly increased the amount of optical fiber connections used within the network in recent years in order to meet the ever-increasing demand from customers, both business and private, for increased broadband data rates. Fiber to the home or premises is the ultimate goal in terms of achievable data rate, but will incur significant costs to the service providers. Instead, hybrid fiber-copper networks have been employed extensively, where the fiber network is extended to a distribution point (DP) in the vicinity of the customer premises, and the existing copper wire infrastructure is used for the last leg of the communications from the DP to the customer premises equipment (CPE).
Recent standards have seen the length of the copper wire drop decreasing as fiber connections are extended to DP's closer to the customer, allowing increases in capacity. Shorter wires offer the potential to expand the region of operation to higher frequency bands, but the channel conditions in these new regions present new challenges.
The MIMO binder channel model has been used in recent standards to model the crosstalk interference between the wire pairs in the binder from DP to CPE. This has allowed cancellation of far-end crosstalk (FEXT) through precoding techniques known as vectoring. In general, the strong near-end crosstalk (NEXT) is avoided through use of time or frequency duplexing. Channel measurements at the higher frequencies now under consideration for next generation digital subscriber line (DSL) systems, show that the FEXT paths provide as much or more power to the receiver than the direct path at these higher frequencies.
These paths make conventional vectoring approaches less effective but offer a source of diversity which may be exploited to improve system performance.